1. Field of the Invention
The present invention relates to an image processing technique and, more particularly, to image processing when a display device displays a moving image.
2. Description of the Related Art
Moving image display devices represented by a TV set can be classified into hold-type display devices and impulse-type display devices. A hold-type display device continues displaying a single image in one frame interval ( 1/60 sec when the frame rate is 60 Hz). A liquid crystal display device and organic EL display using TFTs are known as hold-type display devices. On the other hand, an impulse-type display device displays an image only in the scanning interval of one frame interval so the pixel luminances start lowering immediately after the scanning. A CRT (Cathode Ray Tube) and FED (Field-Emission-type Display) are known as impulse-type display devices.
A hold-type display device is known to have a problem that a viewer readily perceives blurs of a moving object displayed on the screen (motion blurring). To cope with the blurs, the hold-type display device raises the driving frequency of its display to shorten the hold time. For example, Japanese Patent Laid-Open No. 2006-184896 discloses a technique (to be referred to as driving distributing hereinafter) which generates two sub frames from one input frame, that is, a sub frame without a high frequency component and a sub frame containing an emphasized high frequency component, and alternately displays two sub frames generated in correspondence with each frame.
On the other hand, an impulse-type display device is more advantageous in moving image visibility than a hold-type display device. However, since the device emits light only instantaneously in each frame interval ( 1/60 sec when the frame rate is 60 Hz), and repeats light emission at the period of 1/60 sec, a problem of flickering may arise. Flickering is more noticeable on a larger screen, and therefore tends to be a serious problem especially in the recent trend shifting toward display devices with wider screens. The impulse-type display device adopts, as a measure against flickering, a technique of raising the driving frequency of its display.
However, the present inventor found by experiments that when driving distributing raised the frame rate, the sum of waveforms of distributed sub frames and the integration effect by human eye were not always the same. More specifically, it was found that a uniform luminance portion of a frame image sometimes looked as if it changed brightness upon driving distributing.